1. Field of the Invention
The present invention relates to a composite ceramic body in which pores of a porous ceramic body that are opened on the surface of the porous ceramic body are occluded by another ceramic material, a method of manufacturing the same and a ceramic filter assembly.
2. Description of Related Art
Porous material is important and utilized in a very wide range of industrial fields such as for separation and adsorption and as a substrate for catalyst. For instance, in chemical engineering many researches and developments to improve reaction efficiencies by utilizing separation, adsorption and catalyst reactions recently and frequently proceeds using porous material in the reaction system, in the reactor for obtaining the desired production by separation and reaction in the flow process after introducing material fluid into the reaction system. Such a reactor is usually used at a high temperature and at a high pressure, and, therefore, superior thermal, chemical and mechanical durability is required for the porous material for the reactor.
Ceramic materials are used for components requiring high thermal, chemical and mechanical durability. Especially, silicon nitride is used in various heavy-loaded environments since it is superior in mechanical, thermal and chemical stability. The application of a silicon nitride porous body for a filter is expected for separation technique in a severe environment such as at a higher temperature than 300° C. and in an acid or alkali mood.
In use for such as a reactor and a separation device, in general, the porous ceramic body is used for a filter assembly connected to a connector made of heat-resistant dense material such as metal and ceramic. Sealing technique for sealing in the connection of the porous ceramic body and the connector is important to prevent leakage of fluids other thin desired ones for separation. Not only superior mechanical, thermal and chemical stability but also superior capability in sealing at a high temperature and at a high pressure are required especially for sealing for the silicon nitride porous body expected in use at a high temperature and at a high pressure.
Various sealing manners have been suggested. For instance, brazing connection by brazing metal, connection by glass and sealing by a metal ring are useful for sealing at a higher temperature than 300° C. and at a high pressure. Some manners for connection of the porous ceramic body and the connector by a bond such as brazing metal and glass have such a disadvantage that low viscosity of the bond due to high temperature when connecting causes occlusion of pores in other area than the desired sealing area by permeation of the bond into the pores due to capillary attraction of the pores of the porous ceramic body. Remarkable difference in thermal expansion coefficient between the porous ceramic body and the bond permeated into the pores has a disadvantage such that stress due to the difference in thermal expansion coefficient causes cracks in the porous ceramic body at a high temperature. Sealing by a metal ring with a porous ceramic body having low mechanical strength such as a thin porous ceramic body has a disadvantage that causes cracks due to loading on the metal ring for sealing.
To prevent the above disadvantages, filling of another material such as ceramic or heat-resistant metal having substantially equal thermal expansion coefficient into the pores of the porous ceramic body at the connection with the connector to be dense has been suggested. For instance, JP 05-319959 A discloses dense ceramic material obtained by impregnation of polysilazane into the pores of the porous ceramic body, crosslinking and hardening of the polysilazane in the pores, and burning treatment for conversion into ceramic material. The polysilazane used to manufacture the dense (or high-density) ceramic material has a structure selected from the following Structures A to D, and has 200-3000 g/mol in its number-average molecular weight and 100 Pa·s or smaller in its viscosity at the temperature of impregnation.                Structure A: Polysilazane having repetition of substantial units of —[(SiH2)n(NH)r]— (where, n and r have any one of 1, 2 and 3).        Structure B: Polysiloxazane having repetition of substantial units of —[(SiH2)n(NH)r]— and —[(SiH2)mO]— (where, n, m and r have any one of 1, 2 and 3).        Structure C: Modified polysilazane having repetition of substantial units of —[(SiH2)n(NH)r]— (where, n and r have any one of 1, 2 and 3), and a crosslinking bonding —(NH)s— (where, s have 1 or 2).        Structure D: Polymetallosilazane, a reaction product of polysilazane and metal alkoxide.        
The high-density ceramic material disclosed in JP 05-319959 A, regretfully, has a large value of about 4% or 2.2% in its porosity even after five repetitions of a set of impregnation and burning. The connection of the above porous ceramic body with the connector may cause leakage at the sealing in use of such as gas separation and nanofilteration (NF) to separate an object having a size in an order of nanometer.
The inventors of the present invention have found that any porous ceramic body of silicon nitride with polysilazane of any one of the above Structures A to D manufactured by the technique according to JP 05-319959 A cracked in the steps of crosslinking and hardening and burning treatment after impregnation. These cracks are presumed to have been generated due to internal stress of polysilazane impregnated into the pores by reduction in its volume in the steps of crosslinking and hardening and burning treatment. Especially, the porous ceramic bodies for such as gas separation and use for a reactor described above tend to crack due to their low mechanical strength derived from their thin body configured and high porosity of 30% or higher prepared for reduction of resistance in permeation of fluid to be separated. The cracks in the porous ceramic body tend to cause not only remarkable lower performance by leakage of the fluid to be separated but also destruction of the body by growth of the cracks at a high temperature or at a high pressure.
It is therefore an object of the present invention to provide a composite ceramic body that is hard to be cracked and preferably densified, a method for manufacturing the same, and a ceramic filter assembly constituted of the porous ceramic body tightly connected to a filter component such as a connector.